The key part of stanza 11 is the following:

what it utters is its only stock and store,
Caught from some unhappy master

In other words, the speaker, troubled by the appearance of the raven and its repetition of the single word, "Nevermore," reassures himself that the raven doesn't know what it is saying. It is not a sentient being who can respond thoughtfully to what the speaker states or asks. It simply has learned the single melancholy word "Nevermore" from an unhappy former owner, and he repeats it mindlessly.

Nevertheless, despite what he says in stanza 11, the speaker can't convince himself that the bird is simply repeating a word it has learned without understanding. In stanza 12, the speaker asks himself what the bird might mean by repeating the word.

The bird casts a spell over the speaker that his rational mind can't shake. Stanza 11 represents his reasoning mind coming up with a logical explanation for the bird's utterance, an explanation he will almost immediately reject.

Reproduced here:

Startled at the stillness broken by reply so aptly spoken,
"Doubtless," said I, "what it utters is its only stock and store,
Caught from some unhappy master, whom unmerciful disaster
Followed fast and followed faster, till his songs one burden bore,---
Till the dirges of his hope that melancholy burden bore
Of "Never---nevermore."

In the prior stanza, the narrator said that the raven, like hope, will leave him. The raven says "Nevermore".

So to reassure himself, the narrator says that the raven could not possibly have replied to him, that what the raven says is just something an old master taught him (stock and store, meaning 'a stock reply' or a generic answer).. The former master was unhappy and thus spoke only 'nevermore'.